Silicon nitride ceramics and silicon carbide ceramics are easily oxidized at high temperatures and eroded in an environment, in which water vapor is present, as a result of corrosion. There is, therefore, a need to protect erosion when non-oxide ceramics are applied as a gas turbine component, requiring the application onto the surface of a water vapor corrosion resistant layer for that purpose.
A mechanism for improving oxidation resistance has been proposed for silicon nitride ceramics having excellent oxidation resistance in high temperatures, as disclosed in, for example, Japanese Patent Laid-Open No. 6-32658, Japanese Patent Laid-Open No. 5-221728 and Japanese Patent Laid-Open No. 5-208870, in which a rare-earth oxide is added as a sintering aid and the resulting compound is formed on the surface.
Lutetium disilicate (Lu2Si2O7) has a relatively low thermal expansion coefficient, and it is known that this material can remain on a non-oxide ceramics surface even after testing in an actual gas turbine environment. This material has begun to be broadly researched as a candidate material for an environmental barrier coating for non-oxide ceramics.
Regarding non-oxide ceramic structures having a rare-earth oxide silica coating deposited, a rare-earth silicate deposited silicon nitride ceramic structure, with the rare-earth with respect to Y, Yb, Er and Dy, is known as disclosed in, for example, Japanese Patent Laid-Open No. 11-139883, Japanese Patent Laid-Open No. 11-12050, Japanese Patent Laid-Open No. 10-87386, and Japanese Patent Laid-Open No. 10-87364. It is also well known that water vapor corrosion can be effectively suppressed in a static environment when the rare-earth is Lu by depositing lutetium silicate on a silicon nitride ceramics.